`algorithms/mul_redc`: Create sub routine

[github-actions[bot]]

On 2022-06-09 @recmo wrote in f0f0ddb “Merge pull request #126 from recmo/redc”:

Create sub routine

                break;
            }
        }
    }
    if reduce {
        // REFACTOR: Create sub routine
        let mut carry = 0;
        #[allow(clippy::cast_possible_truncation)] // Intentional
        #[allow(clippy::cast_sign_loss)] // Intentional
        for (r, m) in zip(result.iter_mut(), m.iter().copied()) {
            carry += i128::from(*r) - i128::from(m);

From src/algorithms/mul_redc.rs:57